Methods for the formation of black coatings or films on the surface of various metals are currently available. The methods vary with the particular type of metal, e.g., ferrous metals, stainless steels, copper and its alloys, zinc and its alloys, and aluminum and its alloys. The composition of the treatment solution and the treatment conditions vary from case to case. As reported in the background of U.S. Pat. No. 4,931,317, it has been previously known to provide a method of forming a black coating with an aqueous resin containing solution followed by baking to produce a coating of the desired weight. The aqueous solution contains a hexavalent chromium compound, a reducing agent and a water soluble resin.
In the treating of many aluminum or aluminum alloy articles the more common procedure is to first apply a conversion coating such as a chromium based coating and thereafter apply a black paint. The first step creates a corrosion resistant conversion coating. The second step of applying paint is virtually nothing more than a decorative and aesthetic step. Such two step processes require extra equipment and waste disposal procedures. Coatings such as those described in the background of U.S. Pat. No. 4,931,317 also require ovens to bake the coated metal to form the coating thereon.
The '317 disclosure itself teaches a method and composition for the formation of a black coating on the surface of various metals by coating and subsequent baking of a treatment solution containing ferrous metal ions, hexavalent chromium, trivalent chromium and a film forming polymer dissolved or dispersed in water. The black nature of the coating is due to the concentration of hexavalent chromium and metal salts (Co, Fe and Ni) in the solution. This disclosure requires that an air knife be used in the process to remove excess coating material from the surface of the substrate as well as the need for ovens in order to cure the coating after it has been applied to the substrate.
U.S. Pat. No. 5,470,613 discloses the composition and method of forming a single step no rinse black conversion coating. It has been found that this process can not be applied to surfaces of various substrates other than aluminum and its alloys. Also, it has been found that the so-called process solution is not stable and, in the long run, this solution breaks down and/or separates into a polymer and hexavalent chromium paste.